We have started with a brief overview of the observable parameter for the 21-cm signal, namely, its differential brightness temperature. Observation of the signal is done through brightness contrast against the CMB. This provides two ways to detect it - via all-sky averaged “global" signal or statistically through fluctuations in the brightness temperature (PS). The basic principle behind each of these methods has been mentioned. The dominant signal suppressant - astrophysical foregrounds have also been explored, and their influence and possible mitigation strategies have been mentioned. Different instrumental systematics affecting global signal and PS observations have also been described. Despite the enormous number of obstacles, there has been a lot of observational headway in the past decade. The EDGES telescope has reported an apparent trough in the global brightness temperature spectrum at 78 MHz (\(z\sim \) 17), where theoretical models predict the presence of a similar feature [1]}. The depth and width of the detected profile, being deeper and broader than predicted, remains a topic of much research. Nevertheless, this was a significant result in the observational front. On the other hand, the most sensitive operational radio interferometers have placed meaningful upper limits on the signal PS from EoR [2]}, [3]}, [4]}, [5]}, [6]} and post-EoR. These limits have helped constrain astrophysical and cosmological parameters at play during the early epochs (for example, see [7]}, [8]}).
